Station selection apparatus for loran receivers



Dec. 12, 1950 G. D. HULST, JR

STATION SELECTION APPARATUS FOR LORAN RECEIVERS Filed Sept. 28, 1946INVENTOR. George BHHIS; JI.

llllll IMNIL AT TURA/E Y 'a x'ed time relation to each other.

Patented Dec. 12, 1950 ,-ITED STATES PATENT OFFICE George D. Hulst, Jrf,Upper Montclair, N. J., as'- sgnor to Radio" Corporation of America; acore poration of Delaware Application September 28, i946, Serial No.760,139

Claims.v l

My invention relates to a system for producing periodic electricalpulses at selected periodic' rates and is an improvement over the systemdescribed and claimed in application Serial No. 552,146; now Patent No.21,450,358, issued September 28, 19418, filed August El, 1944, in thename of Earl Schoenield and entitled Timing Marker and. Stale tionSelection Apparatus.

The invention is particularly applicable to navigation systems of theLoran type employing pairs of synchronized ground stations that transmitradio pulses having at theV instant of radiation Each pair of grou-ndstations transmits pulses having a different repetition period than thatof the other pairs o stations. The pulses are radiated to receiv-v ingequipment located on the craft whose position is to be determined.

By means or the receiving equipment, the operator on the craftdetermines the time difference between the pulses from the twoktransmitter stations of one pair as they arrive at the receiver. Sincethe radiopulses travel from the ground transmitters to the receiver ata; knownI propa-A gation rate (i. e., at the velocity of light), it isknown that thev position of the craft is at some point on a lnecorresponding to the time difterence reading. By obtaining the time-`diierence reading from a second pair of ground stations a second linecorresponding to the second time dillerence reading is obtained, and theintersect point of the two lines is the position of the craft. Specialmaps having time difference lines printed thereon for the several pairsof ground stations are provided for use with the naviga'- tion system.

My pulse producing system will be described applied to the receiv'ngapparatus in a system of the above-described type. ln such systems, inorder' to measure the time diierence in the arrival of successive pulsesfrom a pair of 'ground stations it is necessary tc produce timing markerpulses that have a known time' :Interval between them. Also, it isnecessary to obtain other pulses having' a denite time relation to thetime marker pulses for the purpose of driving or synchronizingdeflecting circuits. These delecting'circuts produce cathode ray sweeptraces on which the marker pulses and/or the received `ground stae tionpulses appear.

For the purpose or selecting a particular pair of ground stations, the'operator4 must be' able to select-,different repetition periods for'the' drive or synchron'zing pulses wherebythe de'e'ctl ing circuits maybe synchrornzedv with theirs@ ceived pulses from the selected pair ofground stations.

Thus a particular pair of ground stations is selected at the receiverapparatus by turning a station selection switch to the vpositionindicated on the receiver panel for obtaining sweep synchronizing pulseshaving the same repetition period as that of the pulses beingtransmitted from the selected pair of ground stations. Now the reeceived pulses from the selected pair of ground statons can be made toappear stationary on the cathode ray sweep or trace whereas those ree'ceived from the other' pairs' of ground stations will move rapidly alongthe same trace'.

In operation, the pulses from the two transmittel stations of a selectedpair of stations (which pulses will be' referred tov as A and B pulses,respectively) preferably are made to ap= pear on' two parallel cathoderay traces and are brought into alignment or coincidence by movi ing oneof them along its cathode ray sweep trace.- The latter is accomplishedby adjusting the time that the sweep for the said one pulse begins; Byvarious methods itis then possible to count certain timing markersappearing on the cathode ray' sweep'tracc-s to determine the desiredtime d erence'. v

In the system described in the above-nientione'd` Schoenield applicationcount substraeting pulses are fed back to two frequency divider stagesin accordance with the positions of two ganged switches. It was fou-ndthat the count subtraction sometimes failed to function proper1-y due"to the fact that the amplitude of the feed-back pulse had one value foreven station rates and a dilerent value for odd station rates. lt wasalso found that this differenceV in pulse amplitude was caused by achange inthe load on' the pulse circuit when one of the switches wasmoved from a position where it did noticed back pulses to" a posit'onwhere itdid feed back pulses.

An object of the present `invention is to' provide an improved methodofand means for producing pulses having selected repetition periods.

fr-further object of the invention is to provide an improved system forproducing pulses having selected repetiftionperiods wherein, for eachselected period; timing marker pulses are produced having repetition'periods with a decimal relatienshipz- Av still further object ci theinvention is to provide' an improved timing pulse generator com:prisin'g acb of frequency" dividers wherein a number of. d erentrepetition periods' forthe pulses`- he: obtained' and wherein' the 3number of such repetition periods may be greater than the number bywhich any one frequency divider divides the frequency.

A still further object of the invention is to provide an improved pulsegenerator system of the type comprising a chain of frequency dividerswherein the pulse repetition rate is changed by feeding pulses back to aplurality of divider stages to make them lose counts.

The present invention is applied to a system wherein timing pulses areobtained from a chain of frequency dividers which receive signal from astable oscillator and wherein means is provided for substracting countsfrom a plurality of the frequency dividers. In the present example thefrequency dividers in which counts are subtracted are of the countercircuit type wherein a storage capacitor is charged in steps in responseto the application of pulses. The count subtracting circuits comprisemeans for charging the storage capacitors an additional amount inresponse to the application of a pulse from the output of the frequencydivider chain.

According to the present invention one of the ganged switches is inseries in one of the feedback leads and has countJ subtracting pulsesfed through it to a frequency divider stage as in the above-mentionedSchoenfeld system, but the other ganged switch is connected in shuntrelation to the other feed-back connection leading to'another frequencydivider. This shunt connection is such as to ground the feed-backconnection for alternate station selection positions. Thus, countsubtraction pulses are applied to the second-mentioned frequency divideronly when the switch is on an ungroundedV contact point. With thisarrangement there is avoided any wide variation in loading on Vthefeed-back pulse source as the station selector switches are moved todifferent positions since the load in one case is the low impedance of adiode when conducting a feed-back pulse and in the other case is the lowimpedance of the switch to ground. Consequently, the feed-back pulsesare always of substantially the same amplitude and the action of thecount subtraction circuits is reliable.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which the singlegure is a block and circuit diagram of the invention.

Referring to the drawing, a crystal oscillator I produces sine wavevoltag-e of stable frequency which, in the example illustrated, is 100kilocycles per second, the repetition period being microseconds. Thefrequency of the oscillator output may be increased or decreasedslightly by a manual adjustment as indicated at the control knob I I forobtaining a right or left drift of a received pulse on a cathode raysweep trace.

The crystal oscillator I0 drives a blocking oscillator I2 or the like toproduce periodic pulses which, in the present example, also recur at therate of 100 k. c. per second. The repetition period or time intervalbetween successive pulses is, therefore, 10 microseconds.

The frequency of the 10 es. pulses is divided by five by means of asuitable frequency divider I3 such as a second blocking oscillator toproduce lis. pulses. While specific values are being given for theseveral frequency division steps, the invention is not limited to theseparticular values.

The 50 es. pulses are applied through a lead I4 to a frequency dividedIt of the counter type described in White Patent.2,l130l1. It divides 4the frequency by two to produce its. pulses. Also, an additional circuitis provided so that the divider I6 may be made to lose a. count for thepurpose of obtaining a different selected pulse repetition period.

The divider I6 comprises a counter circuit portion including an input orbucket capacitor I1, a pair of diodes I8 and I9, a storage capacitor 2|and a blocking oscillator portion 22. In addition, it includes a pair ofdiodes 23 and 24 associated with the storage capacitor 2I for thepurpose of making the divider I6 lose a count upon the application of apulse from a conductor 26 as will be explained hereinafter. The blockingoscillator 22 comprises arvacuum tube 2l, a transformer 28 coupling theplate circuit to the grid circuit and a cathode biasing resistor 29which bypassed by a capacitor 3l. A transformer 32 supplies the 100 fis.pulses from the divider I6 to a frequency divider 33 which also is ofthe type which may be made to lose a count The frequency divider I6operates as follows: Each'of the 50 lis. pulses of positive polarityfrom the lead I4 puts a predetermined charge on the comparatively largecapacity storage capacitor 2l as a result of a pulse of current throughthe diode I9, the capacity of the capacitor II being small enough sothat capacitor I'I receives full charge before the termination of anapplied pulse. At the end of this current pulse, the capacitor II isdischarged to ground potential through the diode I8. The next 50 es.pulse puts' an additional current pulse into capacitor 2|, this raisingthe voltage across capacitor 2| sufficiently to trigger the blockingoscillator 22 whereby a pulse is produced across the transformer 28 asis well understood in the art. The pulse thus produced is applied to thedivider 33 with positive polarity. At the same time the blockingoscillator 22 discharges the capacitor ZI to bring it back to groundpotential. t

The frequency divider 33 divides the frequency by five to produce 500es. pulses. It includes a counter portion comprising a bucket capacitor36, a pair of diodes 37 and 38, and a storage capacitor 39. It alsoincludes au blocking oscillator portion 4I comprising a vacuum tube 42,a feed-back transformer 43, a biasing resistor 44 and a bypass capacitor46.

As in the preceding divider IB, there is provided in the divider 33 apair of diodes 41 and 48 for subtracting counts. In the divider 33,however, the application of a pulse from a conductor 49 will subtractone, two or three counts depending upon the position of the stationselection switch.

The 500 es. pulses are supplied over a conductor 5I to a frequencydivider 52 that divides by two to produce 1000 as. pulses. The divider52 is similar to the divider I6 with the count subtracting diodesomitted, the circuit constants, of course, differing because of thedifferent frequencies involved.

The 1000 es. pulses are supplied to a frequency divider 56 that dividesby five to produce 5000 ps. pulses which, in turn, are supplied to afrequency divider 59 that divides by four to produce 20,000 ps. pulses..The dividers 55 and 53 are similar to the divider 52 except for thedierence in circuit constants.

The 20,000 es. pulses may be passed through a clipping circuit 60 andsupplied to a square wave generator 6I, such as an Eccles-Jordanoscillator for obtaining a square wave having a repetitionperiodof40,^000 lis. .From this square wave are to the second countsubtraction circuit through a coupling or blocking capacitor or .largecapacity to a second station selection switch 01 which is ganged withthe .switch 64 as indicated by the broken line 68.

At the switch 04, alternate switch `contact points il, 3, and 'i .areconnected to ground so that the feed-back conductor 20 is grounded atalternate switch positions. It will be seen that when on a groundedContact point, switch 60 is connected across or in shunt to .the diode23 and the storage capacitor 2l. Therefore, the 20,000 pls. pulses arefed back to the divider E0 to subtract counts only if the switch 00 ison one Yof the other Contact points, 2, d, E or 8. It may be `desirablebecause of distributed or stray capacity in `the switch 00 to connectits switch arm to ground through a 1 megohin resistor 55 to permitcharges to leak oi.

At the switch 0l, the last sie; switch contact points are connected inpairs, the three pairs of contact 'points 3 0, 5 0 being `connectedthrough bucket capacitors '5L 'i2 and 13, respectively, to the feed-backconductor i0 which leads -to the second count subtraction cir cuit.Thus, with switch @il in any one of the last six positions, 20,000 cs.pulses are applied to the divider 33 to subtract counts.

Before `discussing in detail the of the count subtracting circuits forstatic selection, it may `be noted that the desired t-rning markerVpulses Aare obtained at various points along the frequency dividercircuit. For example, for use with one lparticular system, the cs.pulses are supplied from the blocking oscillator i2 through a delay`network l5 to an output lead 0l. The 50 as. pulses are :supplied to twooutput leads 02 and :83. The 100 its. pulses and the 500 us. pules aresupplied over conductors 04 and 00 to common output :lead f8.1, The 1000as. pulses are supplied to `an output lead 00. The lead 01' and the lead08 vmay :connect to .a common output lead (not shown) whichsupplies the100 its. pulses the 500 .pulses .and the `1.000 as. pules to verticalrle'ectng plates of a cathode ray tube (not shown), the cathode ray ofwhich is deflected horizontally by the waves from the defiecting circuitTl in Ysynchronisn'i with the 40,000 as. square wave It is evident thatthe 10,000 cs. cycle of horizontaldeflection a fixed time relation tolithe 20,000 its. pulses and and the timing marker pulses.

Count Subtraction Referring now more particularly to the .fea- 'ture`oi. subtracting counts for the purpose of station selection, specificpulse repetition rates Jfor a plurality of pairs of ground transmitter`stations will be referred to by way of `example to aid in explainingthe operation.

Itwill 'be assumed that the rst pair of ground stations `transmit the Avpulses with a repetition period of 40,000 us. and transmit the B pulses'with a like repetition period; that the ,pairo ground stations transmitA and B pulses having Aa repetition period -of 39,900 its.; that thecharge applied by a .single 100 as. pulse.

third pair transmits 39,800 as. pulses; that the fourth pair transmitsy39,700 fis. pulses, etc. It is apparent that for station selection atthe receiv ing apparatus, the operator must be able to selectcorresponding repetition periods for the output of the square wavegenerator 6l which controls the cathode ray deflection cycle; namely,periods of 40,000 as.; 39,900 its.; 39,800 frs.; 30,700 its.; 39,600us.; etc.

It will be noted that the several repetition periods differ from eachother by 100 as. or by integral multiples thereof, and that 'thiscorresponds to repetition period diierences of 50 cs. or integralmultiples thereof .at the output of the Yfrequency divider chain, i. e.,at the input ofthe clipper B0. Therefore, the .desired repetition periodcan be obtained by shortening the 20,000 as. period by 50 its., by 100ps., by 150 ts., etc.

For example, to obtain the 39,900 repetition period the switches 5t and0i are moved to the #2 switch contact `points. At this switch positionthe 20,000 fis. pulses from the lead 02 are fed back by way of thebucket" capacitor and the conductor 20 to the frequency divider l0 sincethe switch arm is not grounded. Upon the occurrence of a 20,000 as.pulse, it produces a pulse .of current through the bucket capacitor .03and through the diode 23 to add a charge .to the storage capacitor 2|.At the end of the pulse, the capacitor 03 discharges through the diode2A toits original potential. It will be noted that at switch position #2the frequency .divider 33 does not receive any feed-back pulses. `Byproperly selecting the capacity value of the bucketJ capacitor 63, theadded charge is made equal to the .charge which is added to thecapacitor 2l by a single 50 ys. pulse. Thus, the 20,000 lis. pulsecauses the blocking oscillator 2.2 to nre one pulse earlier or 50 cs.sooner than it normally would whereby the desired repetition period of19,950 lis. at the clipper 60 or 39,900 as. at the output of .the E-J.oscillator 5I is obtained. It may be noted that, in the example given,each time a 20,000 us. pulse occurs, the divider l0 divides by oneinstead of by two.

To obtain the 39,800 as. repetition period, the switches 64 and B7 aremoved to position #3.

'Now the lbucket capacitor 03 is grounded through the Switch 54 .and the20,000 as. pulses are applied through the bucket" capacitor H to thedivider 33 only. The grounding of the arm ofthe switch 64 on the contactpoint 3 puts approximately the same load on the divider 59 as was put onit in switch `position #2 by the conducting diode 23 and the storagecapacitor 2i. Therefore, the pulses `from the divider 50 havesubstantially constant amplitude regardless ot the switch position. Uponthe occurrence of 4a 20,000 ps. `pulse at the divider 59 it applies acharge to the capacitor 39 of v-the divider 33 through the diode 48. Atthe end of the pulse the capacitor 'H discharges through the diode 151to its original potential. The capacitor 'H is Agiven a capacity valuesuch that this charge applied by the 20,000 cs. pulse is equal to theThus, upon the occurrence of a 20,000 cs. puise the blocking oscillatoril res one pulse early or 100 cs. sooner than it normally would wherebythe desired repetition :period of 19,900 cs. is obtained at the clipper00 or .39,800 ,11s. .atthe .E-J `oscillator output. It may be`notedth-atir. the example given, .the `divider 33 divides by `ourinstead of by :five upon vthe 'occurrence `of .reach he.

pulse,

To obtain the 39,700 as. repetition period, the switches 64 and 61 aremoved to the #4 position. Now the 20,000 frs. pulses are applied to boththe divider I6 and the divider 33 whereby both dividers lose -a count.Specifically, the 'blocking oscillators 22 and 4I of dividers I6 and 33fire 50 as. and 100 ps. early, respectively, or a total of 150 ps.early. Thus, the desired repetition period of 2 l9,850 lis. or 39,700as. is obtained at the E-J oscillator output.

To obtainV the 29,600 as. repetition period, the switches l64 and 6l aremoved to the #5 position, this being the switch position shown in thedrawing. Again the 20,000 ,us pulses are applied to the divider 33 only,but this time through the capa-citor 'I2 which has a capacity value suchthat a 20,000 es. pulse causes the divider 33 to lose two counts, i. e.,to trigger 200 ps. early. Thus, the desired period of 2 l9,800 as. or39,600 as. is obtained at the E--J oscillator.

At the #6 switch position, the divider I6 again triggers 50 as. earlyand the divider 33 triggers 200 as. early, or a total of 250 its. forYthe two dividers. Thus, the repetition period is 19,750

.14s. at the input to clipper 60 or 39,500 ps. at the output of the E-Joscillator 6 I At the #7 switch position only the divider 33 receives20,000 as. pulses. These pulses are applied through the capacitor 'I3which is Iadjusted to make the divider 33 lose three counts. Thus, ittriggers 300 lis. early to give a repetition period of 2 19,700 ps. or39,400 as. at the E-J oscillator output.

At the #8 switch position, both of the dividers I6 and 33 lose counts,divider I6 triggering 50 as. early and divider 33 triggering 300 es.early, or a total of 350 fus. whereby the repetition period is 19,650its. at the clipper 60 or 39,300 fis. at the E--J oscillator output.

It may be preferred to employ a different group of repetition periodsthan the group of 40,000 as., 39,900 us., etc. assumed above. By makingthe nal divider stage 59 divide b-y three, for example, instead of byfour, the divider chain output pulses have a repetition period of 15,000es. so that a group of repetition periods of 30,000 ps., 29,900 pis.,etc., may be employed. Or the divider stage 59 may be made to divide byfive to obtain a group of repetition periods of 50,000 fis., 49,900 as.,etc.

In the above-described system the chain of frequency dividers providesthe desired decimal relation timing marker pulses of 10 lis., 100 es.and 1000 ,us in addition to providing the other desired groups ofpulses. The number of repetition periods available in a repetitionperiod group is not limited to the largest number by which the frequencyis divided in a single frequency divider; in the example described thenumber of repetition periods being eight (i. e., 40,000 Ves., 39,900ns., etc., to 39,300 es.) while the largest frequency division step isve. This makes it possible to obtain from the frequency divider chainthe decimal relation pulses in addition to the other desired pulses, andit also improves the tightness of the frequency lock-in because of thesmaller steps of frequency division employed. At the same time theswitching arrangement of the present invention provides a suflicientlyconstant loading on the feedback pulse source to ensure reliableoperation of the count subtraction circuits.

I claim as my invention: Y Y

1. A generator for producing periodically recurring electrical pulses,said generator` comprising a comparatively high frequency oscillator anda chain of frequency dividers connected to receive signal from saidoscillator, two of said dividers including count subtracting means forcausing the divider to lose a predetermined number of repetition periodcounts upon the application of an electrical pulse thereto, a connectionincluding a switch in series therewith for selectively applyingelectrical pulses from the output of said divider chain through saidswitch to the count subtracting means of one of said two dividers, aconnection and a switch in shunt therewith for selectively applyingelectrical pulses from the output of said divider chain'to the countsubtracting means of the other of said two dividers, said shunt switchhaving certain contact points that are grounded and other contact pointsthat are not grounded whereby pulses are fed back to said other divideronly when said shunt switchV is on an ungrounded contact point.

2. A generator for producing periodically recurring electrical pulses,said generator comprising a comparatively high frequency oscillator anda chain of frequency dividers connected to receive signal from saidoscillator, two of said dividers including count subtracting means forcausing the divider to lose a predetermined number of repetition periodcounts upon the 'application of an electrical pulse thereto, aconnection including a switch in series therewith for selectivelyapplying electrical pulses from the output of said divider chain throughsaid switch to the count subtracting means of one of said two dividers,a connection for applying electrical pulses from the output of saiddivider chain to the count subtracting means of the other of said twodividers, a switch connected in shunt with said last connection andhaving alternate contact points that are grounded and having theremaining contact points not grounded whereby pulses are fed back tosaid other divider only when said shunt switch is on an ungroundedcontact point, and means for ganging said two switches.

3. A generator for producing periodically recurring electrical pulses,said generator comprising a comparatively high frequency oscillator anda chain of frequency dividers connected to receive signal from saidoscillator, two of said dividers including count subtracting means forcausing the divider to lose a predetermined number of repetition periodcounts upon the application of an electrical pulse thereto, a connectionincluding a switch in series therewith for selectively applyingelectrical pulses from the output circuit of said divider chain throughsaid switch to the count subtracting means of one of said two dividers,a connection and a switch in shunt therewith for selectively applyingelectrical pulses from the output of said divider chain to the countsubtracting means of the other of said two dividers, said shunt switchbeing open on alternate station selection positions whereby pulses arefed back to said other divider only when said shunt switch is on one ofsaid alternate positions and whereby there is a certain loading on saidoutput circuit when said pulses are fed back, and means for causing saidshunt switch to put at least approximately said certain loading on saidoutput circuit when the shunt switch is on any one of the remainingstation positions.

4. A timing pulse generator comprising a stable oscillator and a chainof frequency dividers connected to receive signal from said oscillator,at least two of said dividers being of the counter circuit type, each ofsaid two dividers including a storage capacitor that is charged in stepsin response to the application of successive pulses, each of said twodividers including a count subtracting circuit, the count subtractingcircuit of the rst of said two dividers including in series with eachother a diode and a capacitor through which the storage capacitor ofsaid rst divider may be charged an additional predetermined number ofsteps in response to the application of a pulse to make said rst dividerlose a predetermined number of counts, a switch connected in shunt tosaid diode and said storage capacitor of the rst divider, said switchhaving alternate contact points that are grounded and having theremaining contact points ungrounded whereby a feed-back pulse chargessaid last-mentioned storage capacitor onlyV when said switch is on anungrounded contact point, the count subtracting circuit of the second ofsaid two dividers including in series with each other a diode, aplurality of capacitors and a switch which may be selectively connectedto any one of said plurality capacitors, the storage capacitor of saidsecond divider being charged an additional predetermined number of stepsin response to the application of a pulse through said lastseries-connected circuit, and each of said plurality of capacitorshaving a different capacity value for causing the count subtractingcircuit of said second divider to subtract different numbers of countsdepending upon which of the capacitors is in said last series circuit.

5. A timing pulse generator comprising a stable oscillator and a chainof frequency dividers connected to receive signal from said oscillatorto produce output pulses, at least two of said dir viders being of thecounter circuit type, each of said two dividers including a storagecapacitor that is charged in steps in response to the application ofsuccessive pulses, each of said two dividers including a countsubtracting circuit, the count subtracting circuit of the first of saidtwo dividers including in series with each other a diode and a capacitorthrough which the storage capacitor of said rst divider may be chargedan additional predetermined number of steps in response to theapplication of an output pulse to make said rst divider lose apredetermined number of counts, a switch connected in shunt to saiddiode and said storage capacitor of the Y rality of capacitors having adifferent capacity Y 10 rst divider, said switch having alternatecontact points that are grounded and having the remaining contact pointsungrounded whereby an output pulse charges said last-mentioned storagecapacitor only when said switch is on an ungrounded contact point, thecount subtracting circuit of the second of said two dividers includingin series with each other a diode, a plurality of capacitors and aswitch which may be selectively connected to any one of said pluralityof capacitors, the storage capacitor of said second divider beingcharged an additional predetermined number of steps in response to theapplication of an output pulse through said last series-connectedcircuit, each of said plu- Value for causing the count subtractingcircuit of said second divider to subtract diierent numbers of countsdepending upon which of the capacitors is in said last series circuit,said switches being ganged and having successive station selectionpositions for applying the output pulses to the second of said countsubtracting circuits through one of said plurality of capacitors for twostation selection positions and for applying said output pulses to therst count subtracting circuit in addition through the capacitor of thenrst series circuit at only one of said two station selection positions,and for applying said output pulses through another of said plurality ofcapacitors for two other station selection positions and through thecapacitor of said rst subtracting circuit in addition at only one ofsaid lastmentioned pair of station selection positions whereby there maybe obtained a plurality of selected repetition periods for said outputpulses.

GEORGE D. HULST, JR.

REFERENCES CITED The following references are of record in the nle ofthis patent:

UNITED STATES PATENTS Number

